1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT), and more particularly to a method and an apparatus for specifying slice planes for obtaining tomographic images in such an X-ray CT.
2. Description of the Background Art
In a conventional X-ray CT apparatus, in order to carry out the multi-scan imaging, the slice planes for obtaining the tomographic images are specified by selecting projection data corresponding to the desired slice planes from a list of projection data displayed on a monitor display, according to the associated data such as the table positions.
Also, in order to carry out the helical scan imaging, the projection data corresponding to the desired imaging region containing the desired slice plane are selected by using the numerical input of the table position and the number of rotations since the start of the scan.
However, in such a conventional X-ray CT apparatus in which the slice planes are specified by selecting the corresponding projection data, precisely which portions of the body to be examined are located at the specified slice planes cannot be confirmed until the reconstructed tomographic image is obtained, so that it has been difficult to accurately specify the slice plane corresponding to the desired tomographic image.
On the other hand, as disclosed in U.S. Pat. No. 4,649,555 for example, there has been a conventional X-ray CT apparatus in which the desired slice planes are specified on a scanogram of the body to be examined prior to the scanning,and then the scans are carried out in order to obtain the projection data necessary in reconstructing the tomographic images for the slice planes specified on the scanogram.
In this type of a conventional X-ray CT apparatus, because the scanning and image reconstruction are carried out after the slice planes are specified on the scanogram, it takes a considerable amount of time for the operator to inspect the reconstructed image in order to determine whether the obtained tomographic image is the desired one, i.e., if the slice planes had been specified appropriately.
Moreover, in a in which the obtained tomographic image is not the desired one, this time consuming process has to be repeated, so that it is not only time consuming but also tedious, and as a consequence there has also been a problem that the amount of X-ray irradiated on the body to be examined is increased considerably in such a case, which is potentially harmful to the body to be examined and therefore highly undesirable.
Similarly, in a case in which this type of a conventional X-ray CT apparatus is used to carry out the helical scan imaging, when the slice planes are specified on the scanogram, the apparatus is controlled by the command indicating the number of rotations before the scanning start and the number of rotations for which the scanning should continue in order to cover the specified slice planes, so that it has been necessary for the operator to inspect the reconstructed image in order to determine whether the obtained tomographic image is the desired one, i.e., if the slice planes had been specified appropriately.
Thus, just as in the case of the multi-scan imaging described above, there have been problems in the helical scan imaging case such that it can be quite time-consuming and tedious for the operator to obtain the truly desired tomographic image, and that a large amount of X-ray is irradiated on the patient.